This invention relates to a method and apparatus for controlling a stepping motor and more particularly is directed to a method and apparatus for electrically dampening a stepping motor.
One of the basic problems with regard to operation of a stepping motor at a high rate of speed is the ability to stop the stepper motor as it arrives at a final position with little or no overshoot or oscillation of the rotor. The prior art discloses a number of approaches to the problem of how to effectively dampen a stepping motor in order to achieve a quick stop time. The two main methods taught by the prior art are by mechanical friction and electrical damping.
Mechanical frictional devices reduce the mechanical overshoot and the audible noise by coupling a rotational inertial mass via a viscous medium. This is usually done by using disc or drums rotationally coupled through a thick silicon oil. These mechanical dampers must be tailored to a specific type motor, and even then they present a limit on the maximum stepping rate due to the load involved.
Electrical dampening has been taught in the prior art in two ways. The first way taught is to distort or correct the driving electrical signal so as to slow the motor during the later portions of each drive pulse. This method is very load-sensitive since the stop or slowdown signal must be changed. A much larger consumption of power is required in this type method and the maximum stepping rate is severely reduced as the full step period would be apportioned between the go and the stop modes. The second method taught in electrical dampening is by the use of dynamic braking. Dynamic braking can be used to utilize the loading of the generator effect of the moving motor to minimize the mechanical overshoot following a stop position change. When the stepper motor makes a step, each coil, whether or not it is energized, produces a voltage by generator action. The overshoot produces a voltage also and this voltage can be clamped or shorted by diodes, resistors, or combinations of both. Unfortunately, stepper motors have, like most multi-phased motors, quite good intercoil coupling, which results in transformer action between driven and undriven coils. The phase of this coupling is such that a drive signal on one coil can be transformed to another coil in a polarity similar to a generated signal on the second coil. This results in the diode or resistor network clamping a transformer coupled drive signal which is undesired because of the power wasted, poor step response, and excessive heat due to the coil being partially shorted during energization of the first coil.